Add builder for BlockingHttp4sServlet

[rossabaker]

Blocking servlets are paleozoic, but:

1. There's a long tradition of them in the servlet world.
2. They can be faster.
3. This works with a mere Sync constraint. Loom might make those fashionable again!

Most people will want an async servlet, of course.

[rossabaker]

I can't remember why this one stalled...

[rossabaker]

I was thinking we could get this old thing mopped up, cut a Servlet 4 and Servlet 5 release (instead of milestones) so they're there, and then quickly bid them goodbye on our way to Servlet 6.